Conventional spark ignition engines are known whose torque is improved by using exhaust pressure pulses. In this type of engines, a valve overlap period during which intake and exhaust valves are kept open is made and negative pressure waves from the exhaust pressure pulses is induced to reach an exhaust port during the valve overlap period over an engine operation region in which torque should be increased, improving the effectiveness of the scavenging action.
For example, Patent Document 1 describes to make phases of the valve overlap period changeable and timing of a negative pressure wave is adjusted so that it reaches an exhaust port during a valve overlap period.
In addition, Patent Document 2 describes to provide valve opening time changing means which makes a valve opening time of an exhaust valve changeable, and to control the valve opening time changing means according to the engine speed so that negative pressure waves from exhaust pressure pulses reach an exhaust port during a valve overlap period, thereby improving the effectiveness of the scavenging action and improving the torque over a wide rotation speed range of the engine.